Induction of Near-vessellessness in Ephedra campylopoda C. A. Mey.

Near-vessellessness was induced in the secondary xylem of Ephedracampylopoda C. A. Mey. by mechanical bark blocking or by wounding.Both treatments resulted in regions of near-vesselless xylem.Xylem formed after the mechanical bark blocking also had regionsin which the orientation of the axial components was changedfrom axial to lateral. Since either mechanical arrest of phloemand cambial transport or wounding of the cambium almost stoppeddifferentiation into vessels, and instead induced differentiationinto tracheids, it seems that the developmental signal for tracheiddifferentiation is not the same as that for vessels. The possibleregulation of near-vessellessness in Ephedra is discussed.Copyright1994, 1999 Academic Press Differentiation, Ephedra campylopoda, near-vessellessness, wood formation, xylem     

Polar auxin transport is implicated in vessel differentiation and spatial patterning during secondary growth in Populus

Premise of the Study

Dimensions and spatial distribution of vessels are critically important features of woody stems, allowing for adaptation to different environments through their effects on hydraulic efficiency and vulnerability to embolism. Although our understanding of vessel development is poor, basipetal transport of auxin through the cambial zone may play an important role.

Methods

Stems of Populus tremula ×alba were treated with the auxin transport inhibitor N‐1‐naphthylphthalamic acid (NPA) in a longitudinal strip along the length of the lower stem. Vessel lumen diameter, circularity, and length; xylem growth; tension wood area; and hydraulic conductivity before and after a high pressure flush were determined on both NPA‐treated and control plants.

Key Results

NPA‐treated stems formed aberrant vessels that were short, small in diameter, highly clustered, and angular in cross section, whereas xylem formed on the untreated side of the stem contained typical vessels that were similar to those of controls. NPA‐treated stems had reduced specific conductivity relative to controls, but this difference was eliminated by the high‐pressure flush. The control treatment (lanolin + dimethyl sulfoxide) reduced xylem growth and increased tension wood formation, but never produced the aberrant vessel patterning seen in NPA‐treated stems.

Conclusions

These results are consistent with a model of vessel development in which basipetal polar auxin transport through the xylem‐side cambial derivatives is required for proper expansion and patterning of vessels and demonstrate that reduced auxin transport can produce stems with altered stem hydraulic properties.     

Xylem pathways in liana stems with variant secondary growth

FISHER, J. B. & EWERS, F. W., 1992. Xylem pathways in liana stems with variant secondary growth. The three-dimensional construction of stem xylem in tropical lianas (woody vines) was studied using several approaches: 1. observations of the xylem surface in stems with bark removed after NaOH treatment or natural retting; 2. reconstructions from serial transverse sections; 3. movement of dye solutions up isolated xylem sectors in intact plants, and 4. flow of dye solutions down branches and xylem sectors in isolated stem segments. Long distance (up to several metres) xylem pathways in unbranched stems and connections between lateral branches and main stems are described for !5 species in eight families which represented seven differnt patterns of secondary growth. The xylem in even the most complex stems is integrated by three-dimensional interconnections of xylem regions which may appear isolated in transverse section. Interconnections are most common at leaf and branch nodes. Some old stems have peripheral xylem that remains isolated over long distances in unbranched stems, but even these had structural and physiological interconnections between xylem regions at branch nodes.     

Diurnal changes in water content of the stems of apple trees, as influenced by irrigation

Abstract Groups of apple trees within an orchard were irrigated by either releasing a fine mist within the canopy or spraying water on the soil. Diurnal changes in the water content of the xylem were inferred from measurements of density made with a gamma probe. Transpiration rates were estimated from leaf temperatures, and water potentials and changes in radial dimension of the stem were recorded. Xylem water content and water potential declined each morning and recovered in the afternoon. Radial dimensions of the xylem followed a similar pattern, but the percentage change was very small in relation to the change in water content of the same tissue. The decline in water content is unlikely to have been caused by cavitation, as it was readily reversed in the afternoon. It is more likely to have been caused by changes in the volume of water occupying already-cavitated fibres or intercellular spaces. Water potential and xylem water content were highest in the mist-irrigated trees and least in the unirrigated controls.     

Ethanol in the stems of trees

Acetaldehyde and ethanol are usually thought to be produced in plant tissues as a mechanism to tolerate hypoxic conditions. We have found acetaldehyde and ethanol to be common in the vascular cambium and in the transpiration stream of trees. In nonflooded trees, acetaldehyde and ethanol concentrations averaged 130 and 40 μ M in the cambium and 130 and 50 μ M in the xylem sap, respectively. Ethanol concentrations in the transpiration stream and the cambium increased to as much as 5 m M upon flooding. Ethanol concentrations in the vascular cambium of Populus deltoides could not be eliminated by placing logs from nonflooded trees in a pure oxygen environment for as long as 96 h, but increased by almost 3 orders of magnitude when logs were exposed to low external partial pressures of O₂. These results suggest that the vascular cambium was not hypoxic, despite the presence of acetaldehyde, ethanol and the enzymes for their synthesis.     

A new method of measuring water potential in tree stems by water injection

Abstract. A new method of measuring xylem water potential in the stems of trees is described. The flow rate of water injected into the xylem at two or more known pressures is measured. The xylem water potential is derived either graphically from the relationship between flow rate and applied pressure, or from the solution of simultaneous flow equations.     

Change in the Concentration and Tissue localization of Endogenous IAA During the Regeneration of Vascular Tissues After Xylem Removal in Broussonetia papyrifera

Change in the concentration of endogenous IAA was monitored with enzyme-linked immunesorbent assay (ELISA), and IAA localization in the tissues was demonstrated by means of immunogold-silver microscopy, with authors' modification, during the regeneration after removal of xylem in Broussonetia popyrifera (L.) Vent. The stimulation exerted from xylem removal produced a rapid increase of about 70% of the endogenous IAA concentration. However, the concentration decreased during differentiation of the vascular tissues. Although removal of the tree crown inhibited the regeneration of vascular tissues, the change tendency in the concentration of endogenous IAA during the regeneration remained the same as if the tree crown was intact. This suggested that rapid increase of endogenous IAA induced by wounding could be a result from a release of the combined form into free IAA. Tissue-localization showed that there were more silver-grains labelled in the rays, callus and the regenerated differentiating vascular tissue cells than in other tissues. It could suggest that the high concentration of endogenous IAA triggered the dedifferentiation of the vascular tissues after removal of xylem, and the less concentrated IAA flow could promote the initiation and activity of the regenerated cambium.     

Effect of Hormones on Embryo After-Ripening of Panax ginseng G. A. Mey. Seeds

Panax ginseng C. A. Mey seed has a deep dormancy for the morphologica and physiological after-ripening of embryo. This paper described the changes of some enzymic activities and of soluble-protein content during after-ripening of seeds, in the first stage of morphological after-development the activities of amylase, peroxidase and catalase increased gradually, whereas the soluble-protein content decresed rapidly. A high level of enzymatic activities and increasing protein content were also observed, indicating vigorous metabolism in physiological after-ripening of seeds. Hormone treatment increased the activities of the above-mentional enzymes, promoted the development and differentiation of embryo, causing the time of seeds germination one year earlier than that of untreated ones.     

Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.     

A Genomic and Molecular View of Wood Formation

Wood formation is a process derived from plant secondary growth. Different from primary growth, plant secondary growth is derived from cambium meristem cells in the vascular and cork cambia and leads to the girth increase of the plant trunk. In the secondary growth process, plants convert most of photosynthesized products into various biopolymers for use in the formation of woody tissues. This article summarizes the new developments of genomic and genetic characterization of wood formation in herbaceous model plant and tree plant systems. Genomic studies have categorized a collection of the genes for which expression is associated with secondary growth. During wood formation, the expression of many genes is regulated in a stage-specific manner. The function of many genes involved in wood biosyntheses and xylem differentiation has been characterized. Although great progress has been achieved in the molecular and genomic understanding of plant secondary growth in recent years, the profound genetic mechanisms underlying this plant development remain to be investigated. Completion of the first tree genome sequence (Populus genome) provides a valuable genomic resource for characterization of plant secondary growth.     

A cytoskeletal basis for wood formation in angiosperm trees: the involvement of cortical microtubules

Rearrangements of cortical microtubules (CMTs) during the differentiation of axial secondary xylem elements within taproots and shoots of Aesculus hippocastanum L. (horse-chestnut) are described. A correlative approach was employed using indirect immunofluorescence microscopy of α-tubulin in 6- to 10-μm sections and transmission electron microscopy of ultrathin sections. All cell types – fibres, vessel elements and axial parenchyma – derive from fusiform cambial cells which contain randomly oriented CMTs. At the early stages of development, fibres and axial parenchyma cells possess helically arranged CMTs, which increase in number as secondary wall thickening proceeds and simple pits develop. In contrast, incipient vessel elements are distinguished by the marking out of sites of bordered pits; these sites first appear as microtubule-free regions within the reticulum of randomly oriented CMTs that characterises their precursor fusiform cambial cells. Subsequently, the ring of CMTs which develops at the periphery of the microtubule-free region decreases in diameter as the over-arching pit border is formed. Like bordered pits, large-diameter, non-bordered pits (contact pits) which develop between vessel elements and adjacent contact ray cells originate as microtubule-free regions and are also associated with development of a ring of CMTs at the periphery. In the case of contact pits, however, there is no reduction in the diameter of the CMT ring during pit development. Tertiary cell wall thickenings are also a feature of vessel elements and appear to form at sites where bands of laterally associated, transversely oriented CMTs, separated from each other by microtubule-free zones, are found. Later, these bands of CMTs become narrower, and separate into pairs of microtubule bundles located on each side of the developing wall thickening. Development of perforations between vessel elements is also associated with the presence of a ring of CMTs at their periphery. Received: 13 July 1998 / Accepted: 30 November 1998